<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            background-color:#fff;
        }
        div{
            width: 0;
            height: 0;
            border:10px solid transparent;
            border-bottom-color:#000 ;
            position: relative;
        }
        div::after{
            content: "";
            display: block;
            width: 0;
            height: 0;
            border: 8px solid transparent;
            border-bottom-color: #fff;
            position: absolute;
            left: 50%;
            margin-left: -8px;
            top: -5px;
        }
    </style>
</head>
<body>
    <div>

    </div>
</body>
</html>